Conventionally, a material such as metal and carbon material has been used in usage where high electrical conductivity is required. Particularly, a carbon material has played an important role in the field of electronics, electrochemistry, energy, transport devices and the like, because this material is excellent in the electrical conductivity, is free from corrosion unlike a metal, and moreover exhibits excellent properties such as heat resistance, lubricity, thermal conductivity and durability. The carbon material has achieved remarkable development also as a composite material comprising a combination of a carbon material and a polymer material, and as a result, such a composite material has played a role of realizing high performance and high functionality. By virtue of the combination with a polymer material, the latitude in mold-processing is expanded and this is one of the reasons why the carbon material has been developed in each field where the electrical conductivity is required.
On the other hand, in view of environmental issue and energy problem, a fuel cell is recently attracting an attention. The fuel cell is a clean power generator of generating electricity by a reverse reaction of electrolysis using hydrogen and oxygen and causing no waste except for water. Also in this fuel cell field, a carbon material and a polymer material can play a large role. The fuel cell is classified into several types according to the kind of the electrolyte but among these, a polymer electrolyte fuel cell can work at a low temperature and therefore, is promising as a power generator for automobile or consumer use. In such a fuel cell, unit cells each comprising, for example, a polymer electrolyte, a gas diffusing electrode, a catalyst and a separator are stacked, whereby high output power generation can be achieved.
In the separator for dividing a unit cell of the fuel cell, flow channels (grooves) for supplying a fuel gas (e.g., hydrogen) and an oxidizing agent gas (e.g., oxygen) and discharging free water (water vapor) generated are generally formed. Therefore, the separator is required to have high gas impermeability capable of completely separating these gases and high electrical conductivity for reducing the internal resistance and furthermore, be excellent in the strength, thermal conductivity, durability and the like.
For the purpose of satisfying these requirements, studies on the fuel cell separator have been heretofore made from both aspects of a metal material and a carbon material. As for the metal material, a noble metal or carbon is coated on the surface with an attempt to solve the problem of corrosion, but sufficiently high durability cannot be obtained and the cost for coating becomes a problem.
On the other hand, many studies have been also made on the carbon material for a fuel cell separator and examples of the separator material for a fuel cell include a molded article obtained by press-molding an expanded graphite sheet, a molded article obtained by impregnating a carbon sintered body with a resin and curing it, a vitrified carbon obtained by firing a thermosetting resin, and a molded article obtained by mixing a carbon powder and a resin and then molding the mixture.
For example, JP-A-8-222241 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (Patent Document 1) discloses a complicated process of adding a binding material to a carbonaceous powder, mixing these under heating, subjecting the mixture to CIP molding (cold isostatic pressing) and graphitization by firing, impregnating the obtained isotropic graphite with a thermosetting resin, applying a curing treatment thereto, and grooving the cured product by cutting work.
Also, JP-A-60-161144 (Patent Document 2) discloses a technique of impregnating a carbon powder- or carbon fiber-containing paper sheet with a thermosetting resin, stacking and press-bonding the paper sheets, and firing the stacked body, and JP-A-2001-68128 (Patent Document 3) discloses a technique of injection-molding a phenol resin in a mold having a shape of the separator and firing the molded article.
The material subjected to a firing treatment as in these examples exhibits high electrical conductivity and high heat resistance but has a problem that the time period required for the firing is long to give rise to poor productivity and also, the flexural strength is low. Furthermore, in the case where cutting work is required, the cost becomes high due to more difficulty of mass production and the prospect as a material to be widespread in the future is very severe.
Studies are being made on a mold-forming method as the means expected to yield high mass productivity and low cost, and the material applicable thereto is generally a composite of a carbonaceous material and a resin. For example, a separator comprising a thermosetting resin (e.g., phenol resin), graphite and carbon is disclosed in JP-A-58-53167 (Patent Document 4), JP-A-60-37670 (Patent Document 5), JP-A-60-246568 (Patent Document 6), JP-B-64-340 (the term “JP-B” as used herein means an “examined Japanese patent publication”) (Patent Document 7) and JP-B-6-22136 (Patent Document 8); a bipolar separator comprising a thermosetting resin (e.g., epoxy resin) and an electrically conducting substance (e.g., graphite) is disclosed in JP-B-57-42157 (Patent Document 9); and a separator comprising a thermosetting resin (e.g., phenol resin, furan resin) having blended therein expanded graphite and carbon black is disclosed in JP-A-1-311570 (Patent Document 10).
In the case of a separator using a composite of a carbon-based filler and a resin, the filling amount of a carbon-based filler needs to be greatly increased for expressing high electrical conductivity but since the resin content is increased so as to maintain the mold-formability, it has been difficult to obtain sufficiently high electrical conductivity.
Furthermore, when the process includes a firing step of heating the molded article at a high temperature of 1,000 to 3,000° C. for obtaining high electrical conductivity, this causes a problem that not only the production takes a long time but also the production process becomes complicated and the cost arises.
The fuel cell separator is also required to have hot water resistance as well as heat resistance. However, the above-described conventional cured product comprising a thermosetting resin and a carbon material fails in having a satisfactory performance with respect to high hot water resistance required in usage as a fuel cell separator. More specifically, the thermosetting resin having an ester or urethane bond in its structure sometimes undergoes hydrolysis due to hot water generated from the fuel cell and therefore, in usage where the operating time is estimated to last long, such as automobile and home appliance, a product having sufficiently high durability cannot be obtained by using a conventional cured product comprising a thermosetting resin and a carbon material.
A separator disclosed in JP-A-2004-250661 (Patent Document 11) is considered to satisfy the performances required of the separator in practical use, such as heat resistance, hot water resistance, durability and mechanical strength, but since unit cells are stacked in the fuel cell and an electric current flows through the separator fundamentally in the penetration direction, if the resistance in the penetration direction is high, the voltage loss and in turn the heating value are, increased and for obtaining a desired high output, an increased number of cells stacked or a large load for the dissipation of heat in an excess heating value is involved. Also from these aspects, higher electrical conductivity is demanded.    [Patent Document 1] JP-A-08-222241    [Patent Document 2] JP-A-60-161144    [Patent Document 3] JP-A-2001-068128    [Patent Document 4] JP-A-58-053167    [Patent Document 5] JP-A-60-037670    [Patent Document 6] JP-A-60-246568    [Patent Document 7] JP-B-64-000340    [Patent Document 8] JP-B-06-022136    [Patent Document 9] JP-B-57-042157    [Patent Document 10] JP-A-01-311570    [Patent Document 11] JP-A-2004-250661